7.3.4.4 Double Input Buffer
The SHA Input Data registers (SHA_IDATARx) can be double-buffered to reduce the runtime of large messages.
Double-buffering allows a new message block to be written while the previous message block is being processed. This is only possible when DMA accesses are performed (SMOD = 2).
The DUALBUFF bit in the SHA_MR must be set to have double input buffer access.